Think its more insidious than any of the answers given already. You have to remember that these are 'natural' stones and as such you need to look back to the time when they were formed. Yes, the Jurassic! And as sedimentary rocks they're an accumulation of material that settled into it. Yes, prehistoric Larva!!
Best if you don't keep them in your sleeping area!
Not good!
( you lapped it didn't you! You exposed a new sediment layer!)